Svitzer Euromed employees have gone on strike for the second time this week over conditions.
Crew union Nautilus International said the tug staff walked out after the Maersk company let an ultimatum expire on working hours.
The action meant tugs sailed reduced hours and did not work until midnight from their usual location in Amsterdam, Nautilus said.
Seafarers want Svitzer to pay 63 staff retrospectively for overtime worked from 1 January.
The union said staff have been working on average 8.5 unpaid hours extra per week since 2014 to help the company out in tough markets.
"Svitzer is in good economic shape, so there is no reason to enforce the crisis measure,’ said Asmae Hajjari of Nautilus partner union FNV Havens.
The unions say Svitzer wants "market-based pay and employment conditions", but refused to pay for the 49 hours currently being worked per week.
Svitzer confirmed to TradeWinds that crews have performed work interruptions of three to six hours on two occasions during the past week.
"We have been negotiating with the union around a new CBA (collective bargaining agreement) for the past eight months, but unfortunately, we have not been able to reach an agreement acceptable for both parties," a spokesperson said.
"Until that happens we apply the terms and conditions of our current agreement.
"We regret the development and would like to see all parties back at the negotiation table to find a sustainable solution for everyone. Currently, we do everything possible to minimise the impacts on our customers."